The book Beyond Earth says we’re looking too close to home for our space colonies.

For a while now, there's been a debate in the US over how to direct NASA's next major human spaceflight initiative. Do we build an outpost on the Moon as a step towards Mars, or do we just head straight for the red planet? Which ever destination we choose, it'll be viewed as the first step toward a permanent human presence outside of the immediate neighborhood of the Earth.

All of that indecision, according to a new book called Beyond Earth, is misguided. Either of these destinations presents so many challenges and compromises that attracting and supporting anything more than short-term visitors will be difficult. Instead, Beyond Earth argues, we should set our sights much farther out in the Solar System if we want to create a permanent human presence elsewhere. The authors' destination of choice? Titan, the largest moon of Saturn.

The case for Titan

Colonizing Titan seems like an outrageous argument, given that the only spacecraft we've put in orbit around Saturn took seven years to get there. Why should anyone take Beyond Earth seriously? Well, its authors aren't crackpots or mindless space fans. Amanda Hendrix is a planetary scientist who's worked at the Jet Propulsion Laboratory and the Planetary Science Institute. For the book, she's partnered with Charles Wohlforth, an environmental journalist who understands some things about establishing a livable environment. And the two of them have conducted extensive interviews, talking to people at NASA and elsewhere about everything from the health complications of space to future propulsion systems.

An interview with the authors about colonizing Titan

The resulting book is a mix of where we are now, which problems need to be solved to make a home elsewhere, and a future scenario that drives us to solve those problems. In this sense, Beyond Earth is a bit like the recent National Geographic effortMars, which blended present-day documentary with a fictionalized future. But the book is a little easier to swallow then the miniseries, which shunted viewers between footage of real-life rockets and CGI dust storms.

Further Reading

So, why Titan? The two closer destinations, the Moon and Mars, have atmospheres that are effectively nonexistent. That means any habitation will have to be extremely robust to hold its contents in place. Both worlds are also bathed in radiation, meaning those habitats will need to be built underground, as will any agricultural areas to feed the colonists. Any activities on the surface will have to be limited to avoid excessive radiation exposure.

Would anyone want to go to a brand-new world just to spend their lives in a cramped tunnel? Hendrix and Wohlforth suggest the answer will be "no." Titan, in contrast, offers a dense atmosphere that shields the surface from radiation and would make any structural failures problematic, rather than catastrophic. With an oxygen mask and enough warm clothing, humans could roam Titan's surface in the dim sunlight. Or, given the low gravity and dense atmosphere, they could float above it in a balloon or on personal wings.

The vast hydrocarbon seas and dunes, Hendrix and Wohlforth suggest, would allow polymers to handle many of the roles currently played by metal and wood. Drilling into Titan's crust would access a vast supply of liquid water in the moon's subsurface ocean. It's not all the comforts of home, but it's a lot more of them than you'd get on the Moon or Mars.

There is the distance thing, which Hendrix and Wohlforth acknowledge, but they argue it's a bit besides the point. The radiation and lack of gravity that make long-range space travel a risk would all bite anyone we sent to explore Mars. NASA assumes it'll find solutions, but the authors are critical of the Agency promoting a journey to Mars without already having solved them. Whether we go to Mars or Titan, the solution is speed: less time in space means less risk. And, if we could rocket along fast enough so that a round-trip to Mars with time spent exploring was safe, then we could do a one-way trip to Titan.

Have some Fi with that Sci

So, Beyond Earth is a good look at the current state of human space-exploration technology, as well as how that will hold us back from doing the things we want to do. It's both thoughtful and thought-provoking.

Mixed in with that, however, is a scenario under which Earth will get its act together and do what needs to be done to overcome these technological hurdles. That scenario is driven in part by a very believable desperation, caused by unaddressed climate change that drives wars and radicalization. Low Earth orbit becomes cheap, and then an efficient new thruster is developed. (Unfortunately, the thruster of choice in this scenario is unlikely to ever work.)

Further Reading

The Earth's governments bands together in a massive effort to send colonists to Titan, who almost immediately begin to view themselves as pioneers who boldly settle a new world with no help from anyone. Tensions and cultural differences ensue. This part of the book is a fun yarn, and plenty of it involves believable extrapolations from our current state. Whether it adds to Beyond Earth overall will probably be a matter of personal taste.

While the focus of the book is on leaving Earth, it's hard to escape the sense that Beyond Earth is an extensive argument for staying put. As Hendrix and Wohlforth repeatedly drive home, there's no place we could go in our Solar System that offers anything close to what the Earth provides for us. Going anywhere else would involve a cost that could go a long way toward making our existence here much more sustainable. While I'm all for eventually establishing a presence elsewhere, it would be nice to do so by choice, rather than end up being forced to do so due to our carelessness on Earth.

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236 Reader Comments

Any serious steps into prolonged space habitation is going to require major construction and assembly facilities that are NOT in Earth's gravity well. The resources we have to expend to get out of said well simply make all goals and objectives far more difficult than they need to be. The Moon is ideal because of the resources already present there, its proximity to Earth in the event of problems, and the greatly reduced gravity, which makes launching anything constructed there that much easier.

The Moon is the best first stop purely from the logistics of what anything and everything else is going to require. Anyone who says otherwise is glossing over many major limitations of our existing space-transportation model.

This is like those weirdos who, whenever we have a spaceX topic, ask why we're not building warp drives instead. Titan is a stupidly long way away (25 times further than Mars if I did my math right) - if we had an Epstein drive, sure, I'd be right there with you. But we don't - Mars itself is barely reachable with current technology in a decent amount of time.

The ability to do really nice aerobraking on both ends of the trip certainly simplifies some aspects of the flight (as does the ease of making fuel and water onsite), but I think that's more than offset by the travel time and less sunlight for solar cells. Surface gravity is even less than the moon, so coming back home is going to seriously SUCK for the astronauts. They'd have to be doing microgravity exercise levels nonstop while on the ground, and even that doesn't help as much as you wish it did.

Titan. Granite-hard ice with rain and rivers of liquid hydrocarbons. If your oxygen mask springs a leak, you risk becoming a walking torch. Mind, if it weren't for the permanently overcast yellow sky, you'd have a lovely view of Saturn.

We should definitely learn how to live on the place. Down the road, that could come in handy (especially if we're island-hopping to the stars via deep-deep-space comets and KBO-like bodies).

But let's get a foothold on those little islands just off shore (Moon and Mars) first, eh?

Mars gives you a very clear atmosphere (most of the time), with not abundant, but useful solar power.

Titan also is energetically very challenging. Solar panels don't work. Winds are mild at best. You've got simple hydrocarbon alkanes everywhere, but they don't react with much and to get energy out of them you have to use oxygen, which you need more energy than that to get from water. Powering a Titan operation is as yet unsolvable. Many things we'll need on Titan need energy, such as hydrolysis and heating and not dying.

However, Titan does have something big going for it. Aerodynamic flight on Mars is nearly impossible. There's so little atmosphere that most aeroplane designs need to exceed orbital velocity at ground level just to get airborne.

At Titan, a crudely sewn wingsuit and the exertion of a jog in the park will get you airborne and keep you there. Until you freeze because you have no way of heating and no energy.

Yeah interesting book and I will probably read it but I can't imagine Titan being the first destination for mankind. The DeltaV to Titan is insane when you consider the thousands of tons of supplies and material you would need to send there.

I think humans will eventually colonize Titan (and every exploitable resources in the system) but it will be decades if not centuries after the Luna and Mars are largely self sustaining.

I can't help but think that any contemporary ideology that could rally world governments and educated, exceptional people into this kind of endeavor -going big instead of starting small- would most likely be seen post-hoc as a reprehensible thing that only ended up eating up massive resources to send people into suicide exploratory missions, considering all the things that can and will most surely would go wrong. In the end having a long-term chilling effect into space exploration.

Maybe the book will change my mind, i'll give it a look.

I don't disagree. I am very worried that we'll eventually end up with a small moon colony that just eats $ and loses support when taxes have to go up 0.5% to pay for it. I don't think a off world settlement would be self sustaining economically for decades if not centuries. I am all for doing what it takes to leave Earth but am super concerned about public support to do so.

With an oxygen mask and enough warm clothing, humans could roam Titan's surface in the dim sunlight.

"enough warm clothing" is a sealed environmental suit with heavy duty heating and lots of insulation. We're talking about an environment where liquid ethane and methane are common, you're not roaming the surface in an oxygen mask and a parka.

Similarly, you're most likely going to be living in tunnels indistinguishable from those used on Mars. You're not going to have big glass domes or Earth-style surface cities, again because of the cryogenic environment. Mars may actually be much better for surface structures, with the right arrangements of shielding.

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And, if we could rocket along fast enough so that a round-trip to Mars with time spent exploring was safe, then we could do a one-way trip to Titan.

...are they even aware that Titan is in orbit around Saturn? Do they know where Saturn is? Travel time to Titan is not going to be twice as long as travel time to Mars. If travel time for a round trip to Mars and back is still a concern, a trip to Titan is out of the question.

Titan is an appealing target for the distant future, extremely rich in useful resources, but many of the supposed advantages they give are simply not real.

However, Titan does have something big going for it. Aerodynamic flight on Mars is nearly impossible. There's so little atmosphere that most aeroplane designs need to exceed orbital velocity at ground level just to get airborne.

The flipside is that the low atmospheric density would make maglev trains just insane. 2000 km/h wouldn't be impossible and would require less energy to overcome air resistance than high speed trains on earth today.

Suborbital rockets using carbon monoxide could be used for long distance travel to areas where train rails aren't built yet. Advantage of carbon monoxide over methane is you could "refuel" using just atmosphere and electrical power. So a self contained "hopper" consisting of suborbital rocket, habitat, and powerplant could go anywhere you need to go on the planet.

I think, if we don't fail here on earth first, we will go to all these places.

I'm pretty sure we're going to fail on Earth and all this discussion is pretty pointless. We're not going anywhere unless we clean up our problems on Earth. And we won't.

We will not have enough resources for meaningful space exploration unless nations of the Earth unite. And that isn't going to happen.

It's pretty obvious to me that humanity is failing.

Failing based on what metrics?

Number of people has more then doubled since WWIIFlight is a little over one hundred years old.Life span effectively doubled since the Victorian ages.Crop yields have increased on less land.Forests are actually increasing in Europe.

During the industrial revolution in Oliver Twists England 40% of children did not survive until five.

As for cleaning up our problems here. The picture of the little blue sphere we call earth has by itself spurred on some pretty significant environmental movements.

Our increased understanding of weather is because of orbital instruments.

On top of that the price point of entry to orbit is diminishing pretty rapidly right now.

However, Titan does have something big going for it. Aerodynamic flight on Mars is nearly impossible. There's so little atmosphere that most aeroplane designs need to exceed orbital velocity at ground level just to get airborne.

The flipside is that the low atmospheric density would make maglev trains just insane. 2000 km/h wouldn't be impossible and would require less energy to overcome air resistance than high speed trains on earth today.

Suborbital rockets using carbon monoxide could be used for long distance travel to areas where train rails aren't built yet. Advantage of carbon monoxide over methane is you could "refuel" using just atmosphere and electrical power.

You wouldn't even need maglev. A regular rail has sufficiently low rolling resistance to offer exemplary performance in the near-absence of atmospheric drag. This is why maglev never really caught on here on Earth: It's better, but only a very little bit better.

A CO rocket also assumes Earth. Where do you get the oxygen from? A process whereby hydrogen is your waste product? Why not just use that?

As long as we're thinking big, why not a plan to terraform Venus? Here are the basic issues:

(1)The atmosphere is 90 times as dense as Earth's and is almost pure carbon dioxide. We could install billions of "clean coal carbon capture units" on balloons in the upper Venusian atmosphere to gradually suck the CO2 down to 200 ppm. Splitting the C off from the O2 to make solid carbon, leaving the O2 behind, would eventually oxygenate the atmosphere. A pure O2 atmosphere would be problematic, however. . . Where do we get the 80% inert nitrogen as on Earth (where it apparently is due to plate tectonics) from?

(2) Once the planet had cooled off a bit after the atmosphere had been stripped of most of the CO2, we could think about the water problem - basically, a whole lot of hydrogen would be needed to mix with the O2 to form water, hence oceans. Here is where the gas giant planets could serve as a hydrogen source. Or, one couse use all the methane on Titan as the hydrogen source.

Okay perhaps that's for another millenia. . . But why aren't we at least building orbital habitats around Venus, the Moon, and Mars? Identical in scale to the ISS, at least, if not much bigger? That is at least a plausible project.

A CO rocket also assumes Earth. Where do you get the oxygen from? A process whereby hydrogen is your waste product? Why not just use that?

CO2 + energy => CO + O2

If there is one thing Mars has is a lot of CO2. NASA has looked at carbon monoxide powered "hopper" drones for Mars because given energy (either solar or RTG) and access to the atmosphere you would have potentially unlimited range. It is relatively low isp but that really isn't a huge concern for a suborbital vehicle.

I don't disagree. I am very worried that we'll eventually end up with a small moon colony that just eats $ and loses support when taxes have to go up 0.5% to pay for it. I don't think a off world settlement would be self sustaining economically for decades if not centuries. I am all for doing what it takes to leave Earth but am super concerned about public support to do so.

By 2030 it's projected China will have a nearly $40 trillion economy, far surpassing the US. It won't be American made rockets or modules colonizing space.

The biggest problem with colonising Mars or Moon or Titan is that there are no particularly compelling reasons to do so, other than being a scifi fan. Work in space is better off left to robots, which are being developed faster than the tech necessary for space colonisation and with no challenges from the laws of​ physics in sight. And if you are willing to live in an underground tunnel, you are much better off doing it here on Earth. The industrial base for building them is all right here. With a bit of cooling, there is no real limit to how far deep one can dig in, negating the limited surface space and the Apocalypse argument. No shortage of energy or raw materials. Or oxigen. Or food. Low ping time, which is crucial for the future digital technology. Shortest possible supply or evacuation path. Standard gravity. And if you change your mind, you can come play outside. Pioneer life on Mars, on the other hand, will be dying of desintery a lot.

As long as we're thinking big, why not a plan to terraform Venus? Here are the basic issues:

(1)The atmosphere is 90 times as dense as Earth's and is almost pure carbon dioxide. We could install billions of "clean coal carbon capture units" on balloons in the upper Venusian atmosphere to gradually suck the CO2 down to 200 ppm. Splitting the C off from the O2 to make solid carbon, leaving the O2 behind, would eventually oxygenate the atmosphere. A pure O2 atmosphere would be problematic, however. . . Where do we get the 80% inert nitrogen as on Earth (where it apparently is due to plate tectonics) from?

I like your thinking. It's wrong, but we can talk about why! We get the nitrogen, the easy bit, because it's already there. Venus has around five times more nitrogen than Earth. Nitrogen is a minority component of Venus' atmosphere, but Venus has MUCH more atmosphere.

The question is what you do with all the excess nitrogen, not how you get it! It's pretty inert and powers a pressure of more than an Earth atmosphere all by itself.

Quote:

(2) Once the planet had cooled off a bit after the atmosphere had been stripped of most of the CO2, we could think about the water problem - basically, a whole lot of hydrogen would be needed to mix with the O2 to form water, hence oceans. Here is where the gas giant planets could serve as a hydrogen source. Or, one couse use all the methane on Titan as the hydrogen source.

Venus will cool off and sink the O2 over about 200 million years, possibly as long as a billion years. (It took Earth that long) At that point, the Sun is a bit too warm.

Venus also has a lot of water. It's a minority component of the atmosphere, but, again Venus has a lot more atmosphere. Earth likely has more, but Venus has some.

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Okay perhaps that's for another millenia. . . But why aren't we at least building orbital habitats around Venus, the Moon, and Mars? Identical in scale to the ISS, at least, if not much bigger? That is at least a plausible project.

Because they need very expensive resupplies every few months and there's no real reason to. The ISS is one of the most expensive megaprojects in history and came out of Cold War cock-waving. It's basically Mir 2 with Americans invited to share the cost.

Isn't the atmosphere and liquid oceans/lakes of Titan made up of hydrocarbons and would be flammable? How would you overcome this? Additionally, this type of atmosphere is also pretty corrosive, right? How do you solve for this? I also think you would need significant machinery to get started. How does this get there? The cost of doing all these things would be enormous.

Isn't the atmosphere and liquid oceans/lakes of Titan made up of hydrocarbons and would be flammable? How would you overcome this? Additionally, this type of atmosphere is also pretty corrosive, right? How do you solve for this? I also think you would need significant machinery to get started. How does this get there? The cost of doing all these things would be enormous.

Well there is no oxygen so combustion isn't possible however if you start bringing oxygen then and have a leak and an ignition source .... BOOM. BADA BOOM.

Isn't the atmosphere and liquid oceans/lakes of Titan made up of hydrocarbons and would be flammable? How would you overcome this? Additionally, this type of atmosphere is also pretty corrosive, right? How do you solve for this? I also think you would need significant machinery to get started. How does this get there? The cost of doing all these things would be enormous.

They burn in oxygen, so oxygen burns in them. Titan has no oxygen.

Reverse the perspective. On Titan, oxygen is highly flammable. Wouldn't all that oxygen on Earth be flammable?

The atmosphere is not corrosive. Without oxygen, alkanes are very unreactive.

A CO rocket also assumes Earth. Where do you get the oxygen from? A process whereby hydrogen is your waste product? Why not just use that?

CO2 + energy => CO + O2

If there is one thing Mars has is a lot of CO2. NASA has looked at carbon monoxide powered "hopper" drones for Mars because given energy and access to atmosphere you would have potentially unlimited range. It is relatively low isp but that really isn't a huge concern for a suborbital vehicle.

It'd be useful for ground vehicles as well...performance kind of sucks relative to LOX/CH4, but manufacture is simpler due to not requiring water. Fuel plants could be strung out along transport routes to refuel ground vehicles.

A CO rocket also assumes Earth. Where do you get the oxygen from? A process whereby hydrogen is your waste product? Why not just use that?

CO2 + energy => CO + O2

If there is one thing Mars has is a lot of CO2. NASA has looked at carbon monoxide powered "hopper" drones for Mars because given energy (either solar or RTG) and access to the atmosphere you would have potentially unlimited range. It is relatively low isp but that really isn't a huge concern for a suborbital vehicle.

Right, gotcha. The engine would, however, erode over time.

There's also the possibility of reacting CO2 with things like magnesium.

Isn't the atmosphere and liquid oceans/lakes of Titan made up of hydrocarbons and would be flammable? How would you overcome this? Additionally, this type of atmosphere is also pretty corrosive, right? How do you solve for this? I also think you would need significant machinery to get started. How does this get there? The cost of doing all these things would be enormous.

Well there is no oxygen so combustion isn't possible however if you start bringing oxygen then and have a leak and an ignition source .... BOOM. BADA BOOM.

You would want to make sure your tunnels were very well sealed.In fact, you're likely going to be tunneling through dirty ice contaminated with all sorts of flammable hydrocarbons. You'll want to be careful about where you let your oxygenated atmosphere get to, and what you bring into your living environments. You'll also want to keep a lot of low oxygen alarms around. Rapid decompression is a bit more noticeable than the breathable atmosphere getting displaced by nitrogen and light hydrocarbons.

As long as we're thinking big, why not a plan to terraform Venus? Here are the basic issues:

(1)The atmosphere is 90 times as dense as Earth's and is almost pure carbon dioxide. We could install billions of "clean coal carbon capture units" on balloons in the upper Venusian atmosphere to gradually suck the CO2 down to 200 ppm. Splitting the C off from the O2 to make solid carbon, leaving the O2 behind, would eventually oxygenate the atmosphere. A pure O2 atmosphere would be problematic, however. . . Where do we get the 80% inert nitrogen as on Earth (where it apparently is due to plate tectonics) from?

I like your thinking. It's wrong, but we can talk about why! We get the nitrogen, the easy bit, because it's already there. Venus has around five times more nitrogen than Earth. Nitrogen is a minority component of Venus' atmosphere, but Venus has MUCH more atmosphere.

The question is what you do with all the excess nitrogen, not how you get it! It's pretty inert and powers a pressure of more than an Earth atmosphere all by itself.

Quote:

(2) Once the planet had cooled off a bit after the atmosphere had been stripped of most of the CO2, we could think about the water problem - basically, a whole lot of hydrogen would be needed to mix with the O2 to form water, hence oceans. Here is where the gas giant planets could serve as a hydrogen source. Or, one couse use all the methane on Titan as the hydrogen source.

Venus will cool off and sink the O2 over about 200 million years, possibly as long as a billion years. (It took Earth that long) At that point, the Sun is a bit too warm.

Venus also has a lot of water. It's a minority component of the atmosphere, but, again Venus has a lot more atmosphere. Earth likely has more, but Venus has some.Okay perhaps that's for another millenia. . . But why aren't we at least building orbital habitats around Venus, the Moon, and Mars? Identical in scale to the ISS, at least, if not much bigger? That is at least a plausible project.

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Venus has enough water to make the surface rock a bit damp (a layer a couple millimeters deep over the entire surface, as I recall), and the reactions required to sink O2 are not likely to happen without more, if they can even sink that much O2 in the first place (and hydration reactions will soak up that film of water a lot more quickly). Terraforming Venus would require importing huge quantities of hydrogen. This would solve both the water and O2 problems, but the quantity of hydrogen required is not a small problem (I forget which SI prefix I worked it out to be in tonnes, but it was one of the bigger ones).

While you're at it, exporting enough CO2 and N2 to give Mars a breathable atmosphere would be a nice side project.

This is like those weirdos who, whenever we have a spaceX topic, ask why we're not building warp drives instead. Titan is a stupidly long way away (25 times further than Mars if I did my math right) - if we had an Epstein drive, sure, I'd be right there with you. But we don't - Mars itself is barely reachable with current technology in a decent amount of time. [/url]. Sold?

No need of Epstein drives, just old-fashioned NERVA fision rockets, from the sixties, would go a long way to reduce those trips travel time.

Q: But why aren't we at least building orbital habitats around Venus, the Moon, and Mars? Identical in scale to the ISS, at least, if not much bigger? That is at least a plausible project.

Quote:

Because they need very expensive resupplies every few months and there's no real reason to. The ISS is one of the most expensive megaprojects in history and came out of Cold War cock-waving. It's basically Mir 2 with Americans invited to share the cost.

Good points on Venus - but the ISS only needs resupply because (1) it has humans constantly aboard and (2) they didn't come up with a recycling system. Ideally such habitats would be constructed and 'manned' by drones and AI systems, not humans (no poop issues), but would be there if humans ever visited or ran into trouble, as waystations of sorts. All the other functions, research mainly, could be conducted by the AI drones.

Venus has enough water to make the surface rock a bit damp (a layer a couple millimeters deep over the entire surface, as I recall), and the reactions required to sink O2 are not likely to happen without more, if they can even sink that much O2 in the first place (and hydration reactions will soak up that film of water a lot more quickly). Terraforming Venus would require importing huge quantities of hydrogen. This would solve both the water and O2 problems, but the quantity of hydrogen required is not a small problem (I forget which SI prefix I worked it out to be in tonnes, but it was one of the bigger ones).

While you're at it, exporting enough CO2 and N2 to give Mars a breathable atmosphere would be a nice side project.

Indeed. Venus and Mars require almost (but not quite) the polar opposite approaches to terraforming. What you take from Venus, you give to Mars -- almost 2 birds with 1 stone. But, that's a project scale beyond anything even the most fanciful of projections could realize (to say nothing of the time scale for it either).

This is like those weirdos who, whenever we have a spaceX topic, ask why we're not building warp drives instead. Titan is a stupidly long way away (25 times further than Mars if I did my math right) - if we had an Epstein drive, sure, I'd be right there with you. But we don't - Mars itself is barely reachable with current technology in a decent amount of time.

The ability to do really nice aerobraking on both ends of the trip certainly simplifies some aspects of the flight (as does the ease of making fuel and water onsite), but I think that's more than offset by the travel time and less sunlight for solar cells. Surface gravity is even less than the moon, so coming back home is going to seriously SUCK for the astronauts. They'd have to be doing microgravity exercise levels nonstop while on the ground, and even that doesn't help as much as you wish it did.

As long as we're thinking big, why not a plan to terraform Venus? Here are the basic issues:

(1)The atmosphere is 90 times as dense as Earth's and is almost pure carbon dioxide. We could install billions of "clean coal carbon capture units" on balloons in the upper Venusian atmosphere to gradually suck the CO2 down to 200 ppm. Splitting the C off from the O2 to make solid carbon, leaving the O2 behind, would eventually oxygenate the atmosphere. A pure O2 atmosphere would be problematic, however. . . Where do we get the 80% inert nitrogen as on Earth (where it apparently is due to plate tectonics) from?

(2) Once the planet had cooled off a bit after the atmosphere had been stripped of most of the CO2, we could think about the water problem - basically, a whole lot of hydrogen would be needed to mix with the O2 to form water, hence oceans. Here is where the gas giant planets could serve as a hydrogen source. Or, one couse use all the methane on Titan as the hydrogen source.

Okay perhaps that's for another millenia. . . But why aren't we at least building orbital habitats around Venus, the Moon, and Mars? Identical in scale to the ISS, at least, if not much bigger? That is at least a plausible project.

Google for the project Colonization of Venus, from Geoffrey Landis.

His idea is to not colonize the surface of the planet, but its atmosphere. Fifty kilometres above the ground has Earth-like temperature, pressure, gravity, light, a 4 days day-night cycle, radiation and micro-meteors protection, .... To take a walk outside you would only need a chemical resistant coat for the occasional droplet of acid.

He imagines colonies on balloons. Considering that our breathable air is a lifting gas on Venus' atmosphere, we would be able to live inside the balloon. Most of the resources for the colonies can be extracted from the atmosphere, and the rest he proposes to lift mineral ore from the surface. And they would be almost unsinkable, because the pressure would be the same inside and outside the balloon, even big tears would be like opening a window on Earth, the air doesn't rush to escape, it mixes very slowly, giving plenty of time to repair the tear.

The biggest problem with colonising Mars or Moon or Titan is that there are no particularly compelling reasons to do so, other than being a scifi fan. Work in space is better off left to robots, which are being developed faster than the tech necessary for space colonisation and with no challenges from the laws of​ physics in sight. And if you are willing to live in an underground tunnel, you are much better off doing it here on Earth. The industrial base for building them is all right here. With a bit of cooling, there is no real limit to how far deep one can dig in, negating the limited surface space and the Apocalypse argument. No shortage of energy or raw materials. Or oxigen. Or food. Low ping time, which is crucial for the future digital technology. Shortest possible supply or evacuation path. Standard gravity. And if you change your mind, you can come play outside. Pioneer life on Mars, on the other hand, will be dying of desintery a lot.

I've always felt that way about robots. We're getting really advanced with AI technology. They seem like the perfect way to really explore the Universe. Barring all the doomsday outcomes, AI will someday be an evolution of human consciousness, but without the mortality and relatively short lifespans that we have.

If we take care of Earth we can live here where the conditions are optimal and have the AI carry on the research in space (and perhaps someday find a perfect second planet for us).

I think we should explore as much as we can. It'll be one of the few redeeming aspects of modernity's ultimate legacy, in which people try to see what was worth it for their ancestors to trash the planet.

Any serious steps into prolonged space habitation is going to require major construction and assembly facilities that are NOT in Earth's gravity well. The resources we have to expend to get out of said well simply make all goals and objectives far more difficult than they need to be. The Moon is ideal because of the resources already present there, its proximity to Earth in the event of problems, and the greatly reduced gravity, which makes launching anything constructed there that much easier.

The Moon is the best first stop purely from the logistics of what anything and everything else is going to require. Anyone who says otherwise is glossing over many major limitations of our existing space-transportation model.

That is one of the reasons space elevators have been tossed around for some time. The ability to transport goods to space at fraction of the cost is something that will be a major boon to humanity.

The downside though is right now we currently do not have a material strong enough and flexible enough to create that massive cable. Carbon nanotubles were one people hoped would be the answer but a single atomic level flaw would reduce its tensile strength in an order of magnitudes.

Though if we can solve that problem, we will have a very effective way of delivering goods on a nearly daily basis and make space exploration a real possibility.